Voltage dependent resistors in a surface barrier type

ABSTRACT

A voltage dependent resistor of the surface barrier type. A sintered body consisting essentially of, as a major part, zinc oxide (ZnO) and 0.05 to 10.0 mole percent of, as an additive, beryllium oxide (BeO), has electrodes in contact therewith. At least one of the electrodes is in non-ohmic contact with the body. The body can have minor amounts of further additives such as nickel oxide, titanium oxide, barium oxide, stannic oxide, aluminum oxide, lead oxide, cadmium fluoride and thallium oxide.

United States Patent Matsuoka et al.

1451 Sept. 5, 1972 VOLTAGE DEPENDENT RESISTORS IN A SURFACE BARRIER TYPEInventors: Michio Matsuoka; Takeshi Masuyarna; Yoshio Iida, all ofOsaka-fu, Japan Assignee: Matsushita Electric Industrial Co.,

, Ltd., Osaka, Japan Foreign Application Priority Data Dec. 8, 1969Japan ..44/98789 Apr. 4, i970 Japan..................45/29908 Filed:Nov. 24, 1970 Appl. No.: 92,380

US. Cl. -3338/20, 317/235 AP, 317/238, 317/235 UA, 252/62.3 ZT Int. Cl...H0lc 7/10 Field of Search ..317/235 AP, 238, 234, 235; 338/20; 252/623ZT [56] References Cited UNITED STATES PATENTS 3,611,073 10/1971Hamamoto ..317/235 3,570,002 3/1971 Masuyama ..317/238 PrimaryExaminer-J. D. Miller Assistant Examiner-Harvey FendelmanAttamey-wenderoth, Lind & Ponack ABSTRACT A voltage dependent resistorof the surface barrier type. A sintered body consisting essentially of,as a major part, zinc oxide (ZnO) and 0.05 to 10.0 mole percent of, asan additive, beryllium oxide (BeO), has electrodes in contact therewith.At least one of the electrodes is in non-ohmic contact with the body.The body can have minor amounts of further additives such as nickeloxide, titanium oxide, barium oxide, stannic oxide, aluminum oxide, leadoxide, cadmium fluoride and thallium oxide.

10 Claims, 1 Drawing Figure PATENTEDSEP 51912 MI CHI O MATSUOKA, TAKESHIMASUYAMA and YOSHIO IIDA,

INVENTORS ATTORNEYS VOLTAGE DEPENDENT RESISTORS IN A SURFACE BARRIERTYPE This invention relates to voltage dependent resistors of a surfacebarrier type and more particularly to varistors comprising zinc oxideand beryllium oxide and having nonohmic electrodes applied thereto.

Various'voltage dependent resistors such as silicon carbide varistors,selenium or cuprous oxide rectifiers and germanium or silicon p-njunction diodes, are known. The electrical characteristics of such avoltage dependent resistor are expressed by the relation:

rent flowing through the resistor, C is a constant equivalent to thevoltage at a given current and exponent n is a numerical value greaterthan 1. The value of n is calculated by the following equation:

n: lO 2/ 1) gic (V2/V1) where V and V are. the voltages at givencurrents I, and 1,, respectively. Conveniently, I and I are 10 mA and100 mA, respectively. The desired value of C depends upon the kind ofapplication to which the resistor is to be put. It is ordinarilydesirable that the value of n be as large as possible since thisexponent determines the degree to which the resistors depart from ohmiccharacteristics.

Silicon carbide varistors are most widely used as voltage dependentresistors and are manufactured by mixing fine particles of siliconcarbide with water, ceramic binder and/or conductive material such asgraphite, pressing the mixture in a mold to the desired shape, and thendrying and firing the pressed body in air or nonoxidizing atmosphere.Silicon carbide varistors with conductive materials are characterized bya low electric resistance, i.e., a low value of C and low value of nwhereas silicon carbide varistors without conductive materials have ahigh electric resistance, i.e., a high value of C and a high value of n.It has been difficult to manufacture silicon carbide varistorscharacterized by a high n and a low C. For example, silicon carbidevaristors with graphite have been known to exhibit n values from 2.5 to3.3 and C-values from 6 to 13 at a given current of 100 mA, and siliconcarbide varistors without graphite show n-valves from 4 to 7 and C-values from 30 to 800 at a given current of 1 mA with respect to a givensize of varistor, e.g., 30 mm in diameter and 1 mm in thickness.

Conventional rectifiers comprising selenium or cuprous oxide have ann-value less than 3 and a C-value of to 10 at a given current of 100 mAwith respect to a specimen 20 mm in diameter. In this case, thethickness of the sample does not affect the C-value.

A germanium or silicon p-n junction resistor has an extremely high valueof n but its C-value is constant, e.g., on the order of 0.3 to 0.7 at agiven current of 100 mA because its diffusion voltage in the V-Icharacteristic is constant and cannot be changed very greatly. It isnecessary for obtaining a desirable C-value to combine several diodes inseries and/or in parallel. Another disadvantage of such diodes is thecomplicated steps involved in their manufacture, with resultant highcost. As a practical matter, the use of diode resistors is not "agedependent resistor, the C-valueof which can be controlled.

These objects are achieved by providing a voltage dependent resistor ofthe surface barriertype having a sintered body consisting essentiallyof, as a major part, zinc oxide (ZnO) and 0.05 to 10.0 mole percent of,as an additive, "beryllium oxide (BeO), and electrodes in contacttherewith. At least one of the electrodes is in non-ohmic contact withthe body. The body can have minor amounts of further additives such asnickel oxide, titanium oxide, barium oxide stannic oxide, aluminumoxide, lead oxide, cadmium fluoride and thallium oxide.

These and other objects of the invention will become apparent uponconsideration of the following description taken together with theaccompanying drawing in which the single FIGURE is a partlycross-sectional view through a voltage dependent resistor in accordancewith the invention.

Before proceeding with a detailed description of the voltage dependentresistors contemplated by the invention, their construction will bedescribed with reference to the aforesaid figure of the drawing whereinreference character 10 designates, as a whole, a voltage dependentresistor having, as its active element, a sintered wafer l ofelectrically conductive ceramic material according to the presentinvention.

Sintered wafer 1 is prepared in a manner hereinafter set forth, and isprovided with a pair of electrodes 2 and 3 having specifiedcompositionsand applied in a suitable manner hereinafter set forth, ontwo opposite surfaces of the wafer.

' The wafer 1 is a sintered plate having any one of various shapes suchas circular, square, rectangular, etc. Wire leads 5 and 6 are attachedconductively to the electrodes 2 and 3, respectively, by a connectionmeans 4 (solder or the like).

According to the present invention, a voltage dependent resistor with ann-value higher-than 5 can be obtained when the resistor comprises asintered body consisting essentially of, as a major part, zinc oxide(ZnO) and 0.05 to 10.0 mole percent of, as an additive, beryllium oxide(BeO) and electrodes in contact with said body, at least one of whichmakes non-ohmic contact.

It has been discovered according to the invention that said sinteredbody 1 has a superior voltage dependent properties when it is providedwith silver electrodes prepared by applying silver paint to oppositesurfaces thereof and firing at to 850 C. in an oxidizing atmosphere suchas air and oxygen. The n-value and C-value of thus produced voltagedependent'resistors vary with the compositions of the sintered body andelectrodes, and their method of preparation. The stability of theresistor with silver paint electrodes is improved when said additiveconsists essentially of 1.0 to 8.0 mole percent of beryllium oxide(BeO).

Since the voltage dependent property of the novel resistorisattributable to the a non-ohmic property of a barrier formed betweensaid sintered body 1 and electrodes 2 and/or 3, it is necessary forobtaining a desirable C-value and n-value to control the compositions ofthe sintered body 1 and the electrodes 2 and 3.

It is necessary for achieving a low'value of'C for the resultant voltagedependent resistors that the sintered body have an electricalresistivity less than ohm-cm, said electrical resistivity being measuredby a four point method in a per se conventional way.

Table 1 shows optimal compositions of sintered body 1 for producing avoltage dependent resistor having an n-value higher than 7 and a highstability with respect to temperature, humidity and electric load.

Table 2 shows operable and optimal compositions of silver electrodes 2and/or 3 after heat treatment.

In the Table 2, the sum of the weight percents of all ingredients shouldbe adjusted so as to be 100 weight percent by controlling the weightpercent of individual ingredients within operable or optimal weightpercents as indicated in the Table.

The sintered body 1 can be prepared by a per se well known ceramictechnique. The starting materials in the compositions defined above aremixed in a wet mill so as to produce homogeneous mixtures. The mixturesare dried and pressed in a mold into desired shapes at a pressure from100 kg/cm ml ,000 kg/cm The pressed bodies are sintered in air at 1,000to l,450 C. for l to 3 hours, and then furnace-cooled to roomtemperature (about to about 30 C). The pressed bodies are preferablysintered in a non-oxidizing atmosphere such as nitrogen and argon whenit is desired to reduce the electrical resistivity. The electricalresistivity also can be reduced by air-quenching from the sinteringtemperature to room temperature even when the pressed bodies are firedin air.

The mixture may be preliminarily calcined at 700 to 1,000 C. andpulverized for easy fabrication in the subsequent pressing step. Themixture to be pressed may be admixed with a suitable binder such aswater, polyvinyl alcohol, etc.

It is advantageous that the sintered body have the opposite surfaceslapped by abrasive powder such as silicon carbide having a particle sizeof 300 mesh to 1,500 mesh.

The sintered bodies are coated at least on one of the opposite surfacesthereof by a silver electrode paint in a per se conventional manner suchas by a spray method, screen printing method or brushing method. It isnecessary that the silver electrode paint have a solid ingredientcomposition as defined in Table 2 after it is fired at 100 to 850 C. inair. Solid ingredients having compositions defined in Table 2 can beprepared in a per se conventional manner by mixing commerciallyavailable powders with organic resin such as epoxy, vinyl and phenylresin in an organic solvent such as butyl acetate, toluene or the likeso as to produce silver electrode paints.

The silver powder may be in the form of metallic silver, or in the formof silver carbonate or silver oxide, or in any other form which infiring at the temperatures employed will be converted to metallicsilver. Therefore, the term silver" as used throughout thisspecification and the claims appended hereto in connection with thesilver composition before it is fired, is meant to include silver in anyform which during firing will be converted to metallic silver. Theviscosity of the resultant silver electrode paints can be controlled bythe amounts of resin and solvent. Particle sizes of solid ingredientsalso are required to be in the range of 0.1 p. to 5 11-.

Lead wires can be applied to the silver electrodes in a per seconventional manner by using conventional solder having a low. meltingpoint. It is convenient to employ a conductive adhesive comprisingsilver powder and resin in an organic solvent for connecting the leadwires to the silver electrodes.

Voltage dependent resistors according to this invention have a highstability with respect to temperature and in a load life test, which iscarried out at C. at a rating power for 500 hours. The n-value andC-value do not change greatly after heating cycles and the load lifetest. It is preferable for achieving a high stability with respect tohumidity that the resultant voltage dependent resistors be embedded in ahumidity proof resin such as epoxy resin and phenol resin in a per sewell known manner.

According to the invention, it has been discovered that the method ofcuring the applied silver electrode paint has'a great effect on then-value of the resultant voltage dependent resistors. The n-value willnot be optimal when the applied silver electrode paint is heated in anon-oxidizing atmosphere such as nitrogen and hydrogen for curing. It isnecessary for obtaining a high n-value that the applied silver electrodepaint be cured by heating in an oxidizing atmosphere such as air andoxygen.

Silver electrodes prepared by any other method than by silver paintingresult in a poor n-value. For example, the sintered body does not becomea voltage dependent resistor when it is provided with silver electrodeson the opposite surfaces by electroless plating on electrolytic platingin a conventional manner. Silver electrodes prepared by vacuumevaporation or chemical deposition result in an n-value less than 3. iThe following examples are given as illustrative of the presentlypreferred method of proceeding accord ing to the present invention;however, it is not intended that the scope of said invention be limitedto the specific examples.

EXAMPLE 1 Respective starting materials according to Table 3 are mixedin a wet mill for 5 hours.

The mixture is dried and pressed in a mold into disc of 13 mm diameterand 2.5mm thickness at a pressure of 340 kg/cm.

Each pressed body is sintered in air at l,350 C. for 1 hour, and thenquenched to room temperature (about 15 to about 30 C). Eachsintered discis lapped at the opposite surfaces thereof lapped by silicon carbidehaving a particle size of 600 mesh. The resulting sintered disc has asize of 10 mm diameter and 1.5 mm thickness. Each sintered disc iscoated on the opposite surfaces thereof with a silver electrode paint bya conventional brushing method. The silver electrode paint employed hasa solid ingredient composition according to Table 4 and is prepared bymixing these ingredients with vinyl resin in amyl acetate. Each coateddisc is fired at 800 C. for 30 minutes in air.

2. A voltage dependent resistor as claimed in claim 1, wherein said atleast one of electrodes consists of a silver paint electrode which is innon-ohmic contact with said body.
 3. A voltage dependent resistor asclaimed in claim 2, wherein said additive consists essentially of 1.0 to8.0 mole percent of beryllium oxide (BeO).
 4. A voltage dependentresistor as claimed in claim 3, wherein said additive further includesat least one member selected from the group consisting of 0.1 to 3.0mole percent of nickel oxide (NiO) and 0.1 to 3.0 mole percent oftitanium oxide (TiO2).
 5. A voltage dependent resistor according toclaim 3, wherein said additive further includes 0.1 to 3.0 mole percentof nickel oxide (NiO), 0.1 to 3.0 mole percent of titanium oxide TiO2)and 0.02 to 1.0 mole percent of barium oxide (BaO).
 6. A voltagedependent resistor according to claim 3, wherein said additive furtherincludes 0.1 to 3.0 mole percent of nickel oxide (NiO), 0.1 to 3.0 molepercent of titanium oxide (TiO2), 0.02 to 1.0 mole percent of bariumoxide (BaO) and 0.1 to 3.0 mole percent of stannic oxide (SnO2).
 7. Avoltage dependent resistor according to claim 3, wherein said additivefurther includes 0.1 to 3.0 mole percent of nickel oxide (NiO), 0.1 to3.0 mole percent of aluminum oxide (Al2O3), 0.1 to 3.0 mole percent oflead oxide (PbO) and 0.1 to 3.0 mole percent of cadmium fluoride (CdF2).8. A voltage dependent resistor according to claim 3, wherein saidadditive further includes 0.1 to 3.0 mole percent of thallium oxide(Tl2O3) and 0.1 to 3.0 mole percent of titanium oxide (TiO2).
 9. Avoltage dependent resistor according to claim 2, wherein said silverelectrode has a composition comprising 70 to 99.5 percent by weight ofsilver, 0.3 to 27 percent by weight of bismuth oxide (Bi2O3), 0.1 to 15percent by weight of silicon dioxide (SiO2) and 0.1 to 15 percent byweight of boron trioxide (B2O3).
 10. A voltage dependent resistoraccording to claim 2, wherein said silver electrode has a compositioncomprising 70 to 99.45 percent by weight of silver, 0.3 to 27 percent byweight of bismuth oxide (Bi2O3), 0.1 to 15 percent by weight of silicondioxide (SiO2), 0.1 to 15 percent by weight of boron trioxide (B2O3) and0.05 to 6.0 percent by weight of cobalt oxide (CoO).